The present invention relates to a process for recovering sulfur from hydrogen sulfide containing gases.
In the conventional sulfur plant, hydrogen sulfide rich gas are processed through the reaction furnace (combustion chamber) followed by the waste heat boiler. The water wall boiler replaces the reaction furnace (combustion chamber) and the waste heat boiler, while the remaining of the equipment stay the same.
It is known that the sulfur present in refinery crudes including natural gas exists as hydrogen sulfide or, in the initial processing steps, is converted in the main to hydrogen sulfide. The hydrogen sulfide, as part of the gas stream, is normally passed through an absorption system, such as an alkanolamine or a physical absorbent, which concentrates it and separates it from other components of the gas stream. The concentrated hydrogen sulfide gas stream is fed to a Claus plant wherein a portion of the hydrogen sulfide is combusted in the presence of oxygen introduced as air, oxygen, or oxygen-enriched air to form sulfur dioxide in proportions for the reaction:2H2S+SO2→3S+2H2O
As fast as sulfur dioxide is formed, it begins to react with hydrogen sulfide in the thermal reaction zone to form sulfur. Sulfur formed is condensed from the gas stream in a waste heat boiler and the balance of the gas stream, at the proper stoichiometric ratio of hydrogen sulfide to sulfur dioxide, is passed to one or more catalytic conversion zone(s), typically three, where additional sulfur is formed by the same reaction. As thermodynamics favor reaction at reduced temperatures, only a limited amount of conversion is achieved in each catalytic bed. The formed sulfur is recovered by condensation and the gas reheated for introduction to a following catalytic bed. The catalysts typically used are alumina or titanium. Conversion efficiencies of from 95% to 97% can be achieved in the Claus plant and, if pollution requirements so dictate, a clean-up operation such as that described in U.S. Pat. No. 3,752,877 to Beavon may be used to increase overall conversion to 99.9%-plus. This operation is also applicable in ammonia burning Claus sulfur plant as described in U.S. Pat. No. 4,038,036 to Beavon.
The oxygen required to convert the hydrogen sulfide to sulfur dioxide is usually supplied with air. This results in the introduction of approximately 79 volumes of nitrogen for every 21 volumes of oxygen needed for oxidation of the hydrogen sulfide. The nitrogen does not benefit the process and actually results in having to use larger and more expensive equipment in the Claus sulfur plant. The amount of nitrogen passing through the plant may be reduced by employing pure oxygen or oxygen enriched air. However, this results in higher temperatures in the Claus reaction furnace.
Temperatures in a Claus unit can reach up to 2800° F. (1538° C.) when air is used in the process and up to 5000° F. (2760° C.) when oxygen is used. A refractory lining insulates the walls of the unit from the high temperature inside of the unit for operation with air. Such refractory linings are undesirable because of the time and expense required to install the lining, the time required to heat the lining during start-up of the unit, the time required to cool the lining during shutdown of the unit, and the expense and lost on-stream time and sulfur production caused by damage to or failure of the lining, which requires that the unit be shut down for repairs.
For high level oxygen operation in the Claus unit, there are no practical refractories capable of withstanding the high temperatures produced in the furnace, therefore, a double combustion process, described in U.S. Pat. Nos. 5,294,428 and 4,780,305, or a recycle process, described in U.S. Pat. No. 4,552,747, is employed to moderate the temperature so that a refractory lining can be used. In the double combustion process, the reactions occur in two stages. Inter-stage cooling is employed in order to not exceed the temperature limit of the refractory. In the recycle process, a portion of the cooled effluent from the reaction furnace is recycled to the reaction furnace to moderate the temperature.
It would be beneficial to design a system to overcome the problems and limitations associated with refractory linings without resorting to two-stage combustion or recycle streams. A water-wall boiler is employed to resolve this limitation.